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Hefty beauty GAGA gets voluptuous new undercarriage

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Our autonomous lawnmower has had a hard summer, with brains, brawn and even her skeleton delayed, but despite failing to manage 2012's growth, she's still on track to wreak graminoid havoc - if just a little later than hoped.

The grass has, quite literally, been growing beneath our feet while we waited for RS Components to make a mockery of delivery promises, and honed our skills in welding and electronics - both of which have proved manifestly inadequate to the task before us - but our autonomous lawn mower, Gaga, now has a body and some muscles to hang off it, and she's turned into a big old girl.

The weight is, in part, due to the motors which RS Components spent almost six months trying to deliver, but also the necessary batteries and decision to use a steel frame, but mostly it's down to a pair of wheels which once graced the undercarriages of RAF Spitfires.

Wheels were still under discussion when Reg reader Stuart Wells offered us a pair of Spitfire tail wheels, which required some redesign, as they were 24cm in height, but the idea was impossible to resist.

It took an overnight soaking in oil, but the wheels did eventually come apart in order to be mounted onto the motors, which finally turned up from RS Components only five months after being ordered.

The Parvalux motors have integral gearboxes, reducing the speed to a garden-prowling-friendly nine revolutions per minute, but took months to arrive as the usually reliable RS Electronics managed to cancel the order (twice), forget to charge us (once), and eventually to deliver three (one too many) Parvalux motors.

After we'd returned one of them we were ready to go, once we had a frame from which to hang all the hardware.

What do you mean you can't weld?

Gaga's frame is constructed with 20mm box steel, welded together with 4mm plates for cross-bracing. The borrowed MIG (Metal Inert Gas) welding rig proved inefficient for the task, mostly because it's located seven miles away at my friend Ian's house, so we splashed out on an ARC rig and starting working out how much we'd forgotten about welding since school.

But despite that, and thanks to the Haynes big book of welding, the frame did eventually come together, allowing us to mount the motors, batteries and connections on a movable platform which more-or-less resembles the design - even if we had to find space for more control circuitry than expected.

It's all a matter of control

When one's previous robotics experience is limited to low-voltage run-arounds requiring (at most) a resistor to slow down the motor, then one is left with an inflated sense of confidence which punctures rapidly as the reality of scaling up such a design dawns. The chaps over at 4QD have a comprehensive guide to this stuff, explaining that connecting a motor directly to a battery risks shorting it out should the motor stall, and controlling speed using resistors is, apparently, just stupid.

Suitably humbled, we plumped for a pair of motor controllers from 4QD, which not only control speed by stuttering the supplied voltage (switching it on and off again at speed) but also ramp the motor up and down as well as preventing it from stalling when we switch to reverse. Ask to go backwards and the controller applies the brakes, feeding the generated voltage back into the batteries, then ramps the speed up in the opposite direction.

All clever stuff, and means we can still use our already-tested relays without having to worry about the effect of slamming Gaga straight into reverse without warning.

Our first attempt to connect up a controller just charred the battery terminal in a shower of sparks, forcing a more careful approach the second time, following the realisation that the battery had been wired up backwards.

"THE CONTROLLER IS NOT POLARITY PROTECTED: BATTERY REVERSAL WILL INSTANTLY DESTROY THE CONTROLLER!" explains the 4QD manual, noting that such damage is not covered by the warranty.

Fortunately the kit is hardier than the manual pretends, and it turns out the shower of sparks is perfectly normal as subsequent testing proved, and the controllers were mounted onto the same frame as the motors along with some basic controls to allow manual control and a big switch to prevent the sparking.

Next up is to imbue the girl with some basic intelligence. While the plan remains to Wi-Fi Gaga up to an in-home server, some readers have suggested that local intelligence would be a good idea, if only to ensure that she shuts down if the network disappears, so we've acquired a very basic brain - just smart enough to have her bouncing off the walls this week.

Once that's working we'll add the already-tested remote control hardware and get back to the bytes and bits, where we're a lot more comfortable. ®